Notes Payable - Earliest Interest Reset Date of Borrowing Outstanding (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Meduim Term Notes Called During the Period	$ 0	$ 0
Due within one year	7,757,050	8,089,826
Long-term Debt, Maturities, Repayments of Principal in Year Two	$ 3,090,405	$ 2,644,734
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.48%	1.91%
Long-term Debt, Maturities, Repayments of Principal in Year Three	$ 2,220,651	$ 1,842,143
Long-term Debt, Maturities, Repayments of Principal in Year Four	859,470	849,263
Long-term Debt, Maturities, Repayments of Principal in Year Five	881,738	790,564
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,434,383	1,306,086
Due after one year	$ 8,486,647	$ 7,432,790
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.10%	1.24%
Debt and Capital Lease Obligations	$ 16,243,697	$ 15,522,616
Debt, Weighted Average Interest Rate	2.30%	1.56%
Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Two	$ 239,327
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.66%
Long-term Debt, Maturities, Repayments of Principal in Year Three	$ 409,629
Long-term Debt, Maturities, Repayments of Principal in Year Four	165,810
Long-term Debt, Maturities, Repayments of Principal in Year Five	253,758
Long-term Debt, Maturities, Repayments of Principal after Year Five	294,720
Due after one year	$ 1,363,244
Due year two [Member] \| Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.13%
Due year three [Member] \| Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.38%
Due year four [Member] \| Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.31%
Due year five [Member] \| Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	3.14%
Due after five years [Member] \| Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	3.27%
Due after five years [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	3.43%
Long-term Debt, Maturities, Repayments of Principal after Year Five	$ 1,173,413
Due year five [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.88%
Long-term Debt, Maturities, Repayments of Principal in Year Five	$ 836,768
Due year four [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.16%
Long-term Debt, Maturities, Repayments of Principal in Year Four	$ 788,504
Due year three [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.32%
Long-term Debt, Maturities, Repayments of Principal in Year Three	$ 1,565,748
Due within one year [Member]
Debt Instrument [Line Items]
Due within one year	$ 9,668,398
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.20%
Due year two [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Two	$ 2,210,866
Long-term Debt, Weighted Average Interest Rate, at Point in Time	1.94%

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